Sensing systems

Abstract

A sensing system for sensing acoustic waves, the sensing system comprises one or more sensor elements for use underwater, a reference sensor, and compensation means for compensating noise generated in the sensing system in dependence upon signals provided by the sensor elements and the reference sensor.

We claim: 
     
       1. A sensing system for sensing acoustic waves, the sensing system comprising: an optical fibre for positioning underwater so as to be exposed to acoustic waves tending to deform the optical fibre, the optical fibre having spaced discontinuities along its length, each pair of adjacent discontinuities with the length of optical fibre therebetween constituting a sensor element;   optical fibre reference means containing light reflecting means;   light generating means for producing coherent light pulses and for transmitting the pulses along the optical fibre and along said optical fibre reference means, the light pulses being reflected from said spaced discontinuities and from said light reflecting means;   detector means for detecting reflected light pulses, the detector means including a demultiplexing means for separating reflected pulses from said optical fibre reference mean into a reference channel, and reflected pulses from said sensor elements into further channel means;   the reference channel including signal processing means to derive a noise phase shift signal representative of phase shift arising from noise within the system, the further channel means including signal processing means to derive sensor element phase shift signals representative of phase shift caused by deformation of the individual sensor elements;   and compensation means responsive to said noise phase shift signal for applying a compensation to the sensing system in order to reduce the effect of noise in said sensor element phase shift signals.   
     
     
       2. A sensor system according to claim 1 wherein the reference sensor comprises a sensor element. 
     
     
       3. A sensor system according to claim 2 wherein the reference sensor is arranged to be acoustically isolated from an acoustic wave received by the optical fibre. 
     
     
       4. A sensor system according to claim 2 wherein the reference sensor is arranged to have a sensitivity to a received acoustic wave which differs from the sensitivity of the sensor elements. 
     
     
       5. A sensor system according to claim 1 wherein the reference sensor comprises an optical fibre element distinct from the optical fibre having the spaced discontinuities and comprising the sensor elements. 
     
     
       6. A sensor system according to claim 1 wherein the demultiplexing means provides a plurality of demultiplexed electrical signals each representing light pulses reflected from different sensor elements. 
     
     
       7. A sensor system according to claim 1 wherein the compensation means comprises a plurality of difference amplifiers for subtractively combining the noise phase shift signal from the sensor element phase shift signals thereby to compensate for the noise generated in the sensing system. 
     
     
       8. A sensor system according to claim 1 wherein the compensation means comprises mixing means for multiplying the sensor element phase shift signals with the noise phase shift signal thereby to compensate for the noise generated in the system. 
     
     
       9. A sensor system according to claim 1 wherein the light generating means comprises a laser source and a piezoelectric device and the generating means is arranged to receive the noise phase shift signal, the piezoelectric device being responsive to the noise phase shift signal thereby to compensate for the noise generated in the system. 
     
     
       10. A sensor system according to claim 1 wherein the light generating means comprises a laser source and a Bragg cell arranged to be driven from a voltage controlled oscillator, and the noise phase shift signal is arranged to control the frequency of operation of the Bragg cell thereby to compensate for the noise generated in the system. 
     
     
       11. A sensing system, the sensing system comprising: an optical fibre for positioning in an environment so as to be exposed to external forces tending to deform the optical fibre, the optical fibre having spaced discontinuities along its length, each pair of adjacent discontinuities with the length of optical fibre therebetween constituting a sensor element;   optical fibre reference means containing light reflecting means;   light generating means for producing coherent light pulses and for transmitting the pulses along the optical fibre and along said optical fibre reference means, the light pulses being reflected from said spaced discontinuities and from said light reflecting means;   detector means for detecting reflected light pulses, the detector means including a demultiplexing means for separating reflected pulses from said optical fibre reference means into a reference channel, and reflected pulses from said sensor elements into respective sensor channels;   the reference channel including signal processing means to derive a noise phase shift signal representative of phase shift arising from noise within the system, each sensor channel including signal processing means to derive a respective sensor element phase shift signal representative of phase shift caused by deformation of respective sensor elements;   and each sensor channel including compensation means responsive to said noise phase shift signal for applying a compensation to the sensor element phase shift signal in order to reduce the effect of noise in said sensor element phase shift signals.